Brake control devices for preventing locking or sliding of vehicle wheels when the brakes are applied by an operator are well known. Such devices have included means for automatically controlling the release and reapplication of the wheel brake. Such "anti-lock" devices have been used in automotive vehicles such as trucks, trailers and buses, as well as in railway cars.
One such anti-locking system relating to trailers is described in U.S. Pat. No. 4,229,051 issued Oct. 21, 1980. A copending application assigned to the same assignee as the present invention, which illustrates and describes a system and sensing valves of the types which may be used in connection with the present invention is "Anti-Locking Mechanism", Pat. No. 4,281,881 issued Aug. 4, 1981.
While the systems described in the aforementioned patent and application have proven satisfactory, the rates at which the sensing valves connect and disconnect the braking pressures during lock up is sometimes not consistent. It is desirable in some cases to control the frequency at which the brakes are switched on and off during "lock up" conditions. Also, it is desirable that the pressures controlling such on and off switching of brakes during "lock up" be relatively constant.